References
U.S. Pat. No. 2,542,057, M. J. Relis, Method and Apparatus for Measuring the Conductivity of an Electrolyte    U.S. Pat. No. 3,806,798 Gross, Electrodeless Conductivity Measurement System    U.S. Pat. No. 7,126,343 Howes, Conductivity Probe with Keeper    U.S. Pat. No. 5,157,332, P. C. Reese, Three-Toroid Electrodeless Conductivity Cell    U.S. application Ser. No. 12/410,443 Samad Seyfi, Apparatus and Method for Measuring Salinity of a Fluid by Inductance    A. Lopes Ribeiro, “Inductive Conductivity Cell for Water Salinity Monitoring”, XVIII Imeko World Congress, Metrology for a Sustainable Development    U.S. Publication 2006/0118472 Karl G. Schick et. al., Single-Use Manifold and Sensors for Automated, Aseptic Transfer of Solutions in Bioprocessing Applications    U.S. Publication 2009/0008060 A1 Samad Seyfi, Apparatus and Method for Measuring Salinity of a Fluid by Inductance    U.S. Publication 2007/0008060 A1 Thomas M. Weller, Total Fluid Conductivity Sensor System and Method    U.S. Pat. No. 2,774,239 E. R. Fitzgerald, 1956, Apparatus for Determining Dynamic Mechanical Properties of Viscoelastic Materials    U.S. Pat. No. 6,586,939 Otto N. Fanini, 2003, Method and Apparatus for Reducing the Effects of Parasitic and Galvanic Currents in a Resistivity Measuring Tool    U.S. Publication 2008/0258935 John K. Quackenbush, Non-Metallic Flow-Through Electrodeless Conductivity Sensor and Leak Detector    U.S. Pat. No. 7,581,434 Frederick M. Discenzo 2009, Intelligent Fluid Sensor for Machinery Diagnostics, Prognosis and Control    U.S. Pat. No. 7,285,420 Fredrick M. Discenzo, 2007, Fluid Sensor Fixture for Dynamic Fluid Testing    U.S. Pat. No. 3,404,335 R. J. Kidder, 1968, Apparatus for Measuring Electrical Conductivity of a Conducting Medium Capable of Flowing in a Conduit    U.S. Pat. No. 7,126,343 Ronald Bruce Howes et. al., 2006, Conductivity Probe with Toroid Keeper
Conductivity of a fluid provides important information about its contents. For example, conductivity of water provides an indication of ionizable constituents of the water, such as salts. This has applications in process control, environmental remediation, and monitoring of water handling systems such as for salt and fresh water. The invention described herein relates generally to an instrument used for measuring the electrical conductivity of fluids. In this type of instrument, a magnetic inductor generates a magnetic field around a core, which may include a ferromagnetic material. This inductor-core combination is typically a electromagnetic coil or winding of toroidal shape, having an inner and outer circumference, as is well known to those in the art. The fluid for which one desires to measure conductivity is permitted into the volume inside the inside circumference of the toroids. A second toroidal coil senses the magnetic field generated by the first toroidal coil by means of magnetic coupling based on the electrical conductivity of the fluid and generates an electrical output related to the electrical conductivity of the fluid.
Electrodes have been used to directly measure conductivity in a liquid by applying a DC or AC voltage between a pair of electrodes and measuring current flow in the path. This has several disadvantages, such as electrolysis and corrosion changing the contact resistance of the electrodes, causing substantial calibration drift, and different behavior of loss mechanisms with frequency of excitation in the case of an AC signal. It can also be challenging for extreme situations like highly corrosive or ultrahigh purity fluids.
For this reason, electrodeless systems for conductivity measurement are commonly used such as that described in U.S. Pat. No. 2,542,057, issued on Feb. 20, 1951 to M. J. Relis for Method and Apparatus for Measuring the Conductivity of an Electrolyte. In that system and others which follow two toroids with multiple windings on each are arranged with some separation such that they align axially. An excitation alternating current voltage is applied to one coil, the drive, primary or excitation coil, which induces a magnetic field inside the toroid. The magnetic field then imposes a current in the fluid such that the electrical current flow path goes through the dual toroid assembly out and around the exterior and back into the center of the coils. The second toroid, referred to as the secondary or sense toroid is used as a sensing circuit. The current through the fluid produces a magnetic field in the sensing toroid. This magnetic field induces a current in the windings of the sensing toroid. The magnitude of the voltage across the sensing toroid is a function of the primary's voltage, water conductivity, and windings on both the primary and secondary.
Improvements such as the use of ferromagnetic cores and opposing windings, described in U.S. Pat. No. 3,806,798, issued on Apr. 23, 1974 to T. A. O. Gross for Electrodeless Conductivity Measuring System have been made since then. The two toroids are also typically encapsulated such that the region in the center and, in some cases such as in a bath or tank, the region outside of the toroids are exposed to the fluid under test but the fluid is prevented from going in between the two toroids or into the windings of the toroids.
As described in U.S. Pat. No. 5,157,332, issued on Oct. 20, 1992 to P. C. Reese for Three-Toroid Electrodeless Conductivity Cell, one can also use two outer drive coils and one inner sense coil located between the drive coils, which helps confirm by switching between drive coils that no fluid has penetrated the toroid cavity or windings. This patent also briefly describes a way of dealing with another, more subtle problem. The literature on double toroid devices of this type states that the toroid is very efficient and has self contained flux lines, so toroids can be mounted in very close proximity and have no cross talk. This turns out not to be completely true. The windings of the toroids capacitively couple from the primary toroid to the sensing toroid and impose a voltage on the sensing toroid. This results in inaccurate readings or even the complete inability to measure meaningful signals for low conductivity fluids. While this patent states that the primary shielding of the signal receiving coil is provided by the surrounding drive coils, metallic shields are also used to reduce the unwanted direct toroid-to-toroid coupling. These shields are described as “magnetic”, however, so would not necessarily act to eliminate the capacitive coupling between drive and sense coils. In addition, in theory, a single ground plane with a shared ground between the two toroids such as described in that patent provides perfect shielding, but in reality, the ground has some impedance and therefore couples some of the high drive current from the primary side to the secondary side. This causes additional electrical noise in the sense coil and thus in the output of the conductivity sensor if multiple shields are all joined to a common ground, as in the Reese patent. Similarly, in U.S. Pat. No. 6,586,939 Fanini, issued in 2003 for “Method and Apparatus for Reducing the Effects of Parasitic and Galvanic Currents in a Resistivity Measuring Tool”, although multiple grounds and shields are discussed they are in the context of preventing electrical cross-talk between the drive and sense toroids which is carried through the outer conductive casing of their conductivity measurement system. No mention is made of shielding between the toroids in order to reduce electrical parasitics due to coupling between the toroids, as occurs with a nonconductive casing.
In U.S. Publication 2007/0008060 A1 by Weller, “Total Fluid Conductivity Sensor System and Method”, a PCB is used for the conductivity sensor, but as the coils are formed out of vias and microstrips on and within the PCB rather than with wires and ferromagnetic cores the PCB is acting as a structural material, and no conductive planes are used as shields between coils to reduce cross-talk. Their device is also not well suited to measurement of a broad range of solutions and conductivities, as it relies on a specific resonance.
By their nature, these conductivity measurement systems operate in an environment in close proximity to fluids which may be of different temperatures. Many parts in the systems may change their operating parameters as a result of changes in temperature. U.S. Publication 2006/0118472 Schick et. al., among others, discusses use of a temperature sensor exposed to the fluid being measured in order to perform temperature compensation of the apparatus. Increased circuit stability and performance can be obtained, however, by also measuring temperatures of components within the system directly. In particular, measurement of the temperature of the toroids as well as fluid temperature can improve circuit stability and performance.
Calibration of such an apparatus is necessary, both to establish initial performance of a particular device after construction and later in the field as components and environmental conditions change. U.S. Publication 2008/0258935 Quackenbush for “Non-Metallic Flow-Through Electrodeless Conductivity Sensor and Leak Detector” discloses using a conductive loop through the toroid openings with a single resistor for calibration of the system. Such a calibration system, however, presumes that any calibration curve will be a straight line through the origin. In the majority of real world measurement situations, a minimum of both a slope and intercept are needed to describe data, which requires 2 point measurement and 2 resistors of different values to be used for calibration. In many cases a curve defined by 3 or more points and 3 or more resistors may be necessary to fully describe a calibration data set.
Amplification of signals from the sense coil is also necessary for these systems. In U.S. Publication 2009/0008060 A1 by Samad Seyfi, a system for using resonance of the inductance of a gap in a toroid is described. While this method provides very high sensitivity, it applies only to a very narrow salinity range because it is tuned for a specific resonance, so does not solve the problem of measurements over a wider range of salinities. The ideal way to carry out amplification is to perform this operation as close as possible to where the signals are detected, i.e. the sense toroid, rather than by using wires carrying these low level signals long distances. This requires use of circuitry components near said toroid. While U.S. Publication 2006/0118472 Karl G. Schick et. al. discusses use of a PCB board in such a conductivity measuring system, the components described are merely present for measurements and data storage, and all data processing takes place outside in a user interface or controller system. There is no indication of signal processing, calculations, calibrations etc. taking place in the sensor area, where signals are strongest.
One way of boosting effective signal strength is by performing a differential signal comparison between the signal and sense toroids. U.S. Publication 2007/0008060 A1 by Weller, for “Total Fluid Conductivity Sensor System and Method” describes making such a comparison by extracting and comparing the imaginary components of the signals from the phase information in the signal. Such a method, however, loses the real part of the signal, which provides the conductivity. Comparing the real part of the signal would allow determination of conductivity from the amplitudes of the signals, which allows for operation at lower frequencies.
Thus, previous work in this field fails to address several issues with these types of conductivity probes. First, capacitive coupling between the toroids is not completely compensated, particularly for low conductivity fluids. The drive coil has a high level AC signal which may vary in frequency, amplitude, waveform, or duty cycle, causing corresponding variation in the magnetic field generated by this toroid. The sense coil reacts to the magnetic field as coupled by a conductive fluid, but electrostatic coupling also takes place between the drive and sense toroid, causing unwanted spurious signal from which the desired signal due to magnetic coupling must be extracted. Second, the signal coming from one or more sensing coils used in the probe is a “low level signal”, in the millivolt range, making it easily susceptible to interference as it travels from the region of the toroidal sensor(s) to electronics which can be used to amplify it for readout by equipment or an operator. Third, depending on the method by which the toroids are sealed from fluid entry, when pressure on the sensor changes (as may happen if it is immersed in fluid), positioning of the toroids can change, causing changes in its response. Finally, assembly of these structures does not take full advantage of recent availability of integrated electronics and sensors which can improve ease of manufacturing and reduce costs to make such a conductivity sensor.